Okay, this took me a while to track down as the source of the problem, but I am out of options here and at my wits end. Every conversation I see even close to this is at best 2 years old, most 2010 or older, so I am desperate for help.
-- radGrid on the page, GridClientSelectColumn set as first column, grud set to have AllowMultiRowSelection="true"
-- after multiple items selected, radwindow used to allow user to edit selected item(s)
If at any time after the window appears (whether it closes or not) any button that does a postback - partial or full - always throws a "Unable to cast object of type 'System.Int32' to type 'System.String'." error. I remove the "AllowMultiRowSelection="true"" (or otherwise set it to "false") everything works perfectly. If it is true, I always get the error, specifically when the grid refreshes. It does not fire any server-side code; the code dies well before it even gets to that point. The same happens is I set a radbutton to be AutoPostback="false" but then try to refresh the grid using client code; if I skip refreshing the grid completely, but I must refresh the grid each time to ensure I have the latest data on it.